More and more consumer electronic products come equipped with one or more cameras to achieve image-recognition functions. Thus, images captured by the cameras need to be clear. Since not every camera is equipped with autofocus features, during manufacturing and assembling in the factory, the camera needs to be adjusted to an appropriate focal length suitable for intended use. However, judging whether the focal length of the camera is optimal is mainly determined by manual inspections, and the focal length is manually adjusted. Manual inspections and adjustments are prone to errors due to factors such as fatigue of the operators. Such a manufacturing method of cameras consumes human resources and production time which may be reduced.